Rashi: They meant that they would not receive a portion on the west side of the Yarden, seeing as they had already received their inheritance on the east side.

Ramban #1 and Moshav Zekenim #1: It was in the form of a request ¹ that they declined to receive a portion in Eretz Cana'an, since they were already living in territory that suited their needs ² - in the hope that Moshe would give it to them.

Ramban #2 and Moshav Zekenim #2: Whether Moshe gave them a portion now or not, they were prepared to relinquish their rights to a portion in Eretz Cana'an in order not to detract from the portions of the other tribes, and they would later return to the land in which they were now living and claim it as theirs - since none of the other tribes wanted it.